The present invention relates to improved green strength of unvulcanized elastomeric compositions containing reclaimed rubber therein. More specifically, the present invention relates to the improvement of green strength by adding semi-crystalline butene polymers or interpolymers thereof to elastomeric compositions containing reclaimed rubber made by devulcanizing vulcanized scrap rubber using mechanical energy, heat, and/or chemical agents, or microwave agents.
The present invention also relates to blends of reclaimed rubber, devulcanized by microwave energy, with crystalline or semi-crystalline polyolefins such as polyethylene or polypropylene. These uncured blends are easily molded into articles which have good resistance to attack by aromatic solvents, aliphatic solvents, and fuel-alcohol mixtures.
A major deficiency of reclaimed rubber as well as reclaimed rubber-elastomer blends is lack of sufficient green strength required for satisfactory fabrication of complex products such as tires and industrial goods. The abatement of this deficiency would greatly facilitate the partial or complete replacement of more costly virgin elastomers with reclaimed scrap (that is, commonly discarded) rubber.
The term "green strength" while being commonly employed and generally understood by persons skilled in the rubber industry is, nevertheless, a difficult property to precisely define. Basically, it is that property of a polymer, common in natural rubber, which contributes the proper building characteristics where multiple components are employed and which result in little or no relative movement of the assembled components subsequent to assembly and prior to initiation of the curing operation. "Tack" is also an important property but the lack of tack is usually readily overcome by the addition of well known and conventional tackifying agents. Green strength, that is adequate mechanical strength for fabricating operations necessarily carried out prior to vulcanization is lacking in many elastomers. That is, the stress at break is low. Thus, unvulcanized strips or other forms of many elastomers often pull apart in a taffy-like manner during building operations. Although numerous additives and compounds have been utilized in association with various elastomers and particularly synthetic cis-1,4-polyisoprene, adequate improvement in green strength has generally not been accomplished.
Green strength has generally been measured by stress/strain curves of unvulcanized compounds. Usually, the performance of a green compound is based upon the ultimate or breaking tensile. Improvement indicates improved green strength.
Among the various additive compounds or agents which have been utilized to improve green strength of synthetic elastomers are numerous nitroso compounds as set forth in U.S. Pat. Nos. 2,457,331; 2,447,015; 2,518,576; 2,526,504; 2,540,596; 2,690,780; and 3,093,614. Additionally, various dioxime compounds have been utilized such as those set forth in U.S. Pat. Nos. 2,969,341; 3,037,954; 3,160,595; and British Pat. No. 896,300. Yet another class of additives or compounds is the diesters of 5-norbornene as set forth in U.S. Pat. Nos. 3,817,883 and 3,843,613.
As set forth in Canadian Pat. No. 870,053, minor amounts of trans-1,4-polyisoprene are used to improve green strength. However, no suggestion is made as to the use of polyolefins, as to the use of elastomer-reclaim rubber blends, or as to the use of reclaimed rubber.
In an article by Kumbhani, "Butyl Rubber--a Useful Elastomer Modifier for Polyolefins," Polysar Limited, 1977, data is set forth substantiating the title. No mention is made of reclaimed rubbers, improved green strength, or the like.
A Romanian article was published in Materials Plastics No. 10 (11), 604,607 (1973) entitled "INFLUENCE OF ADDITIONS OF POLYBUTYLENE WITH DIFFERENT MOLECULAR WEIGHTS ON THE PROPERTIES OF COMPOSITIONS OF CIS-POLYISOPRENE SYNTHETIC RUBBER" and prepared by B. Mehr and T. Volintiru. This article discloses the use of polybutylene which is mixed with synthetic or natural rubber to give increases in various properties. As set forth on page 2 of the translation of the article, the molecular weight must be low and cannot exceed 10,000 since otherwise a mixture with a synthetic rubber cannot be obtained. The article does set forth data which shows that various cured physical properties are improved such as the increase in resistance to repeated bending and increased ozone resistance. However, as plainly evident by the table set forth on page 3 of the translation, no appreciable increase in uncured tensile or rupture strength is obtained when polybutene is utilized. In fact, the bottom of page 3 of the translation clearly states that in comparison with low pressure polyethylene utilized as a mixture, the polybutylenes do not improve the green strength of cis-polyisoprene synthetic rubber. Thus, this article is not pertinent.
Another U.S. patent, namely U.S. Pat. No. 3,909,463 assigned to Allied Chemical Corporation, relates to the preparation of graft copolymers wherein a synthetic rubber is grafted onto an olefin polymer backbone such as polypropylene and polybutylene whereby a composition is formed which is free of substantial amounts of crosslinked rubber. The amount of olefin polymer utilized is from 40 percent to about 99 percent by weight. Additionally, a bifunctional phenol-aldehyde condensate is utilized in forming the graft copolymer. The grafted copolymer formed appears to be a high impact resin. Thus, this article is also not relevant to the present invention.
The present application is readily distinquished from the above reference in that a physical blend is formed as opposed to a chemical reaction for forming a graft copolymer, a rubber compound is formed as opposed to a high impact resin, low amounts of polybutene are utilized in comparison to the high amount contained in the graft copolymer and no bifunctional phenol-aldehyde condensate is utilized whatsoever in the present invention.
U.S. Pat. No. 3,654,197 to Seifert relates to molded substances, having improved cold resistance and resistance to stress cracking, which are formed from mixtures of partially crystalline, partially amorphous, polymers together with small but significant amounts of sulfur-vulcanizable unsaturated rubber and sulfur. However, this patent relates to essentially thermoplastics which are crosslinked and contain very low amounts of unsaturated rubber, that is up to 20 percent maximum. No suggestion of using reclaimed rubber is made.
In contrast, applicant's invention relates to elastomeric blends containing a minimum of 40 parts by weight of elastomer including the incorporation of reclaimed rubber therein, with such blends having improved green strength. Thus, the Seifert reference is inapplicable.
It is noted that U.S. Pat. Nos. 3,123,583 to Howard; 3,965,055 to Schichman; 3,701,702 to Schichman; 4,005,054 to Bonnefon; British Pat. No. 946,224 to Burke; British Pat. No. 1,120,572 to Asahi and the article by Mehr and Volintiru, "Materials Plastics" No. 10 (11), 604-607 (1973); were cited as merely being of interest in the parent application since they generally fail to show applicant's improved green strength elastomers. Moreover, these references generally fail to suggest the use of reclaimed rubber.